1. Field of the Invention
The present invention relates generally to the field of batteries and, more specifically, to separators containing glass fibers which are positioned between the positive and negative plates of batteries and to a method for producing such separators. As is subsequently discussed in more detail, separators containing glass fibers are well known. Long before glass fiber separators, however, cedar veneers were used as a separator material, and were replaced by microporous, hard rubbery separators and cellulose separators impregnated with resins.
2. Description of the Prior Art
Valve regulated (xe2x80x9csealedxe2x80x9dxe2x80x94xe2x80x9crecombinantxe2x80x9d) lead acid (VRLA) batteries are known; they usually comprise a plurality of positive and negative plates, as in a prismatic cell, or layers of separator and positive and negative electrodes wound together, as in a xe2x80x9cjelly rollxe2x80x9d cell. The plates are arranged so that they alternate, negative-positive-negative, etc., with separator material and paste separating each plate from adjacent plates. The separator, which, typically, is a mat of glass fibers, is an inert material; it stores battery acid, applies a force to paste-grid interfaces, and provides low electric resistance. In addition, in VRLA batteries, there are innumerable gas channels in the separator material through which oxygen can migrate from the positive electrode, when generated there, to the negative electrode where it can be recombined with hydrogen, according to the oxygen cycle. One of the most important functions of a separator in a VRLA battery is to force the paste into contact with the plates, and cause a pressure between the plates.
Glass fiber separator material, typically, is produced commercially on paper making equipment including fourdrinier machines and rotoformers, inclined fourdrinier machines and extended wire rotoformers. In the production of separator made of glass fibers for VRLA batteries, it is preferred that no organic material be added to a furnish from which separator sheets are made; the entanglement of individual fibers serves to maintain the sheet in a cohesive structure, and water glass, which sometimes forms on the fiber surfaces, serves as a binder. Organic binders, however, tend to decrease the ability of a separator to wick acid, and to decrease the amount of acid a separator can hold. A great deal of work has been directed to modifying the glass fiber furnish from which separators are produced to improve battery performance and/or lower the cost of the separator. Some of the work has entailed the addition of synthetic fibers for various reasons, such as the use of thermoformable plastic fibers so that the separator can be heat sealed on its edges to envelop a plate. Other work, which pertains to the field of this invention, has been directed to the use of filler, e.g., silica, to provide separators which are comparable to all glass fiber separators, at a lower cost. Separators made from glass fibers to which cellulose has been added and polyolefin fibers to which cellulose has been added have also been suggested. Prior art patents are discussed below.
U.S. Pat. No. 4,465,748 (Harris) discloses glass fiber sheet material for use as a separator in an electrochemical cell, and made from 5 to 35 percent w/w of glass fibers less than 1 xcexcm in diameter; the patent also discloses a glass fiber sheet for such use wherein there are fibers of a continuous range of fiber diameters and lengths, and most of the fibers are not over 5 mm in length.
U.S. Pat. No. 4,216,280. (Kono et al.), discloses glass fiber sheet material for use as a plate separator in a battery, and made from 50 to 95 percent w/w of glass fibers less than 1 xcexcm in diameter and 50 to 5 percent w/w of coarser glass fibers. The coarser glass fibers, the reference says, have a fiber diameter larger than 5 xcexcm, preferably larger than 10 xcexcm, and it is advantageous for some of the coarser fibers to have diameters of 10 xcexcm to 30 xcexcm.
U.S. Pat. No. 4,205,122 (Minra et al) discloses a battery separator of reduced electric resistance comprising a self supporting, non woven mat consisting essentially of a mixture of olefinic resin fibers having a coarseness of from 4 to 13 decigrex and olefinic resin fibers having a coarseness of less than 4 decigrex, the latter fibers being present in an amount of not less than 3 parts by weight per 100 parts by weight of fibers; up to about 600 parts by weight of inert filler materials per 100 parts by of fibers can also be used. The battery separator is produced by subjecting a suitable aqueous dispersion to a sheet-forming operation, drying the resulting wet, non-woven mat, and heat treating the dried mat at a temperature ranging from a point 20xc2x0 lower than the melting point of the aforementioned fibers to a point about 50xc2x0 higher than the melting point.
U.S. Pat. No. 4,216,281 (O""Rell et al.) discloses a separator material produced from a furnish containing 30 to 70 percent w/w of polyolefin synthetic pulp, 15 to 65 percent w/w of a siliceous filler and 1 to 35 percent w/w of xe2x80x9clongxe2x80x9d fibers which can be polyester fibers, glass fibers, or a mixture of the two. Cellulose in an amount up to about 10 percent w/w is disclosed as an optional ingredient of the furnish.
U.S. Pat. No. 4,363,856 (Waterhouse) discloses a separator material made from a furnish composed of polyolefin pulp fibers and glass fibers, and names polyester staple fibers, polyolefin staple fibers and cellulose pulp fibers as alternative constituents of the furnish.
U.S. Pat. No. 4,387,144 (McCallum) discloses a battery separator having a low electrical resistance after extended use which is made by thermal consolidation and thermal embossing of a paper web formed from a furnish containing a synthetic pulp the fibrils of which are filled with an inorganic filler, the web incorporating a wetting agent which is preferably an organic sulphonate, and organic succinate, or phenol ethoxylate.
U.S. Pat. No. 4,373,015 (Peters et al.), discloses sheet material for use as a separator in a battery, and xe2x80x9ccomprising organic polymeric fibersxe2x80x9d; both of the examples of the reference describe the sheet material as xe2x80x9cshort staple fiber polyester matting about 0.3 mm thickxe2x80x9d, and indicate that the polyester fibers range from about 1 xcexcm to about 6 xcexcm in diameter.
Sheet separators for use in conventional (not valve regulated) batteries and comprising both glass fibers and organic fibers are disclosed in all of the following U.S. Pat. No. 4,529,677 (Bodendorf); U.S. Pat. No. 4,363,856 (Waterhouse); and U.S. Pat. No. 4,359,511 (Strzempko).
U.S. Pat. No. 4,367,271, Hasegawa, discloses storage battery separators composed of acrylic fibrils in an amount of up to about 10 percent w/w, balance glass fibers.
Japanese patent document 55/146,872 discloses a separator material comprising glass fibers (50-85 percent w/w) and organic fibers (50-15 percent w/w).
U.S. Pat. No. 4,245,013, Clegg et al., discloses a separator made by overlaying a first sheet of fibrous material including polyethylene fibers with a second sheet of fibrous material including polyethylene and having a synthetic pulp content higher than the first sheet.
U.S. Pat. No. 4,908,282, Badger, discloses a separator comprising a sheet made from first fibers which impart to the sheet an absorbency greater than 90% and second fibers which impart to the sheet an absorbency less than 80% wherein the first and second fibers are present in such proportions that the sheet has an absorbency of from 75 to 95%. This patent discloses that fine glass fibers have a high absorbency, that coarse glass fibers have a low absorbency, and that hydrophobic organic fibers have an extremely low absorbency, and that, when this separator is saturated with electrolyte, unfilled voids remain so that gas can transfer from plate to plate for recombination. The disclosure of Badger is incorporated herein by reference.
U.S. Pat. No. 5,091,275 (Brecht et al.) discloses a glass fiber separator which expands when exposed to electrolyte. The separator comprises glass fibers which are impregnated with an aqueous solution of colloidal silica particles and a sulfate salt. The separator is produced by forming a paper making web of glass fibers, impregnating the web with the aqueous mixture of silica and the salt, lightly compressing the impregnated web to remove some of the aqueous solution, partially drying the web, compressing the web to a final thickness and completing the drying of the web. The web is preferably compressed to a thickness which is less than the distance between plates in a given cell, so that insertion of an assembled cell stack into a case is facilitated. When electrolyte is added to the case, the salt dissolves in the electrolyte and the separator expands to provide good contact between the plates and the separators. According to the patent, the silica contributes to the recombination performance of cells incorporating the pre-compressed separator. The silica also contributes a great deal of stiffness to the separator, so much so that the separator may be characterized as rigid.
It has been determined that the production of battery separator by paper-making techniques from a furnish of glass fibers and silica powder leads to problems which are caused by variations in the concentration of the silica powder in the furnish. Typical glass fiber furnishes have a liquid content exceeding 98 percent w/w. In the course of making separator sheets, most of the water is removed from the furnish in the first few feet of a screen on which the furnish is cast. The water, known as white water, is recycled and winds up back in the headbox of the machine. If the furnish is composed exclusively of glass fibers, virtually none of the fibers pass through the wire and wind up in the white water. However, furnishes comprising glass fibers and silica powder do not fare so well. In the absence of a retention aid, significant amounts of silica powder from such furnishes do pass through the paper making wire and wind up in the white water. Left unchecked, this phenomenon causes the concentration of silica powder in the furnish to increase, undesirably changing the properties of the furnish. Heretofore, the problem of silica powder and the like passing through a paper making wire has been avoided through the use of binders as retention aids.
U.S. Pat. No. 2,477,000 discloses a synthetic fiber paper produced from fibrillae and fibers made by methods wherein a solution of the fiber is extruded through very small orifices (spinnerets) and then the extruded solution is allowed to congeal either in a precipitating bath or by evaporation of the solvent or by temperature changes (see column 2, lines 25 and following). The patent says that fibers of cellulose acetate, cellulose nitrate, regenerated cellulose from viscose, xe2x80x9cVinylite (a synthetic resin made by polymerization of vinyl compounds), Aralac (a fibrous product made from skim milk casein), and spun glassxe2x80x9d which range in length from _inch to 1 inch and in diameter from 12-80 microns and fibrillae preferably derived from flax, Manila hemp, caroa or hemp can be used to make the paper. At least 90 percent of the fibrillae should be from 0.0015 to 0.0025 inch in length and from 0.0000027 to 0.0000044 inch in width.
The instant invention is based upon the discovery that comparatively small additions of wood pulp, if beaten or refined to a sufficient degree to produce a highly fibrillated cellulose fiber, to a glass fiber furnish suitable for use in making battery separator material,
(1) cause surprisingly high increases in some of the strength properties of separator made from the furnish,
(2) improve the cut through resistance of a separator made from the furnish,
(3) and have a unique characteristic in that they hold a greater proportion of acid introduced thereunto when the separator is subsequently compressed.
In addition, the separator is repulpable, in the sense that it can be used as a constituent of a glass fiber which is used to produce xe2x80x9cnewxe2x80x9d separator: furthermore, batteries made from glass fiber separator material which contains comparatively small amounts of wood pulp which has been beaten or refined to a sufficient degree, have remarkably long service lives, as indicated by their performance in cycling tests. In general, the pulp slurry should be beaten or refined to a Canadian freeness not greater than about 650 cc, or to an equivalent freeness by other measurement techniques, and a remarkable increase in tensile strength is achieved when the pulp is beaten or refined to a Canadian freeness not greater than about 120 cc, or to an equivalent freeness by other measurement techniques.